In a line transfer type of CCD imager, the image register comprises a plurality of paralleled CCD charge transfer channels in which charge packets representative of a radiant energy image are caused to form. The charge packets in each CCD channel represent a line of image response, and these lines of image response are sequentially transferred in respective charge packet trains from the image register. Transfer may be to a common read-out bus connecting the output electrodes for each of the image registers. A disadvantage of using such a read-out bus is that the output signal-to-noise ratio is diminished because of the high capacitance of the long read-out bus. Alternatively, the common read-out bus may be replaced by a CCD charge transfer channel running alongside the imager register, its successive stages arranged for side-loading from respective ones of paralleled charge transfer channels in the image register. The side-loaded CCD charge transfer channel introduces a time skew between the lines in excess of that associated with their being regularly scanned, so delay compensation must be provided to avoid shear distortion in a television image reproduced from the imager output signal.
Alternative ways of reading out the contents of the imager register in a line-transfer CCD imager are described in a U.S. patent application Ser. No. 650,615 entitled "TDM-Input Electrometer, as in a Line Transfer CCD Imager, Using a Charge Funnel", filed Sept. 14, 1984 by P. K. Weimer, and assigned to RCA Corporation. These alternative ways involve the use of a "charge funnel". A charge funnel is a CCD charge transfer channel, wide enough at its input port to connect directly from the parallelled output ports of several or all of the charge transfer channels in the image register, and narrow enough at its output port that a floating-element electrometer provides substantial response to charge level variation at that port. Weimer describes the division of the image register into bands of adjacent charge transfer channels, the output ports of the charge transfer channels in each of these bands being followed by a respective charge funnel. Weimer describes the plurality of charge funnels being dynamically clocked on a continuous basis and in synchronism with each other.
There are a number of advantages in using a plurality of smaller charge funnels in the line transfer CCD imager, rather than one larger charge funnel, for conveying charge packets from the image register to the electrometer. The rate at which the width of the charge funnel can narrow is constrained by the need to keep the effective length of charge transfers small, so that charge transfers are as complete as possible, in order to avoid image lag. So the minimum length of a charge funnel is determined by the width of its input port. The parallelly disposed input ports of the plurality of smaller charge funnels are narrower than the input port of the single charge funnel they replace, so the length of each of them can be shorter. The shorter length of each of the plurality of smaller charge funnels means there is less time delay through them. This facilitates the changes in image register clocking, when different lines are selected for forward charge transfer, not coupling to the electrometer input during line trace portions of the electrometer output signal. This places the line selection artifacts in the line retrace intervals, so they can be suppressed by blanking the electrometer output signal line during line retrace.
The reduction of charge funnel length, together with the width of its input port, causes the area taken up on imager die by the plurality of charge funnels to be smaller than that area that would be taken up by the single charge funnel. The smaller area of the plurality of smaller charge funnels means that the overall capacitance between gate electrodes and substrate is smaller. So less power is required to clock charge packets through the plurality of smaller charge funnels. However, there is still a desire to further reduce clocking power dissipation in the line transfer CCD imager camera.